The overall performance of a 100-pixel superconducting tunnel junction (STJ) array detector in the soft X-ray region was determined by measuring fluorescent X-rays from light elements. The yield of working STJs in the fabricated detector was 90%, and the leakage current for these 90 STJs was 11.6 ± 2.5 nA. The STJs had an intrinsic energy resolution for O-Kα X-rays (524.9 eV) of 10.2 ± 1.7 eV (FWHM). The deviation from energy linearity was −0.042 ± 0.45%, 0.793 ± 0.91%, and 0.135 ± 1.31% for O-Kα (524.9 eV), N-Kα (392.4 eV) and B-Kα (183.3 eV) Xrays, respectively. Most of the STJs exhibited uniform X-ray detection characteristics. In addition, the detector could discriminate the N-Kα peak in the fluorescent X-ray spectrum from n-type SiC doped with 300 ppm nitrogen.